Magnetically soft materials are used widely as magnetic cores and electromagnetic shielding materials; such as in transformers, magnetic heads, and magnetic sensors. Such materials are required to have suitable characteristics for thir respective uses. For example, higher magnetic permeability is desired for magnetic head materials, and smaller iron loss is desired for transformer materials. Magnetically soft materials are also used, for example, for sensors in which pulse voltages are abruptly generated by combined use of an electromagnetic induction type pick-up coil. The materials exhibiting large Barkhausen discontinuity, which undergoes extremely rapid magnetic reversal, are very useful for such pulse generating elements and are widely utilized.
The "large Barkhausen discontinuity" referred to above is a phenomenon of instantaneous magnetic reversal in a material caused by application of a critical magnetic field, which results in an extremely steep linear rise in the magnetic hysteresis loop, namely a jump in magnetization. In a magnetization process, a typical magnetic substance is magnetized uniformly and continuously with increase of the intensity of an applied magnetic field. In contrast, a substance exhibiting a large Barkhausen discontinuity is magnetized discontinuously on applying a magnetic field. Barkhausen discontinuity is frequently observed in typical magnetic substances as a step-like noise. This phenomenon is believed to be due to abrupt movement of magnetic domain walls resulting from disappearance of restrictions on their movement caused by grain boundaries in the magnetic substance when the intensity of the applied magnetic field is increased. This Barkhausen discontinuity is observed as a step shape when the magnetic hysteresis loop is magnified, and the jump in magnetization is not so large.
H. Barkhausen, who first identified the Barkhausen discontinuity in 1919, reported that a single domain causing one of the Barkhausen discontinuity reversals has a size of approximately 10.sup.-8 cc [H. Barkhausen: Phys. Z. 20 (1919) 401]. On the contrary, in a large Barkhausen discontinuity, a far larger portion in comparison with usual Barkhausen discontinuity, several % to several ten % of the specimen is simultaneously magnetized and oriented, exhibiting extremely steep discontinuous magnetization in the magnetic hysteresis loop. Since no formal definition exists regarding the lower limit of large Barkhausen discontinuities at present, the phenomenon of discontinuous magnetization exceeding 10% of the saturation magnetization will be defined as "large Barkhausen discontinuity" in the present invention.
The large Barkhausen discontinuity is observed, at the moment of application of a critical magnetic field having intensity H*, as a jump in magnetization caused by abrupt movement of magnetic walls. Practically, the movement of the magnetic walls, and the speed thereof, can be observed and measured, which is one of the characteristics of the large Barkhausen discontinuity phenomenon. In other words, the large Barkhausen discontinuity may be confirmed by observation. For example, the movement of the magnetic domain walls and the velocity thereof are directly observable according to the methods reported by K. J. Sixtus and L. Tonks (Phys. Rev., Vol. 37, (1931) p. 930; Vol. 39 (1932) p. 357; Vol. 42 (1932) p. 357; vol. 43 (1933) p. 70 and p. 931; and so forth.
As shown in FIG. 2 in above-cited Phys. Rev. Vol. 37 (1931), p. 930, a specimen is magnetized in one direction by means of a main coil capable of applying a magnetic field to the whole specimen to give it a single domain structure. Thereafter, the electric current flowing through the main coil is gradually decreased to zero, and then the magnetic field is applied in the reverse direction with gradual increase up to slightly lower level than the critical magnetic field intensity H*. By this process, the specimen is first magnetized in a single magnetic domain structure; and, subsequently, is subjected to a magnetic field in the direction reverse to that of the first magnetization. At this stage, if a magnetic field of intensity H* is applied by means of an additional coil placed at the end of the specimen in the same direction as that of the main coil, reverse magnetic domains are formed at the position of the additional coil, and the magnetic domain walls move extremely rapidly toward the other end of the specimen.
This movement of the magnetic domain walls is detected as pulse voltages by two search coils I and II placed at intermediate positions along the length of the specimen. The velocity of movement of the magnetic domain walls is measured by the time lag of the voltage induction at the search coils I and II, which are placed at separate positions. The number of magnetic domains having moved between the search coil I and the search coil II can be counted by the number of the generated pulses. In the case of the large Barkhausen discontinuity phenomenon, only one pulse, instantaneous long-range movement of only one magnetic domain wall, can be detected by suitably controlling the magnetic field created by the main coil and the additional coil. Therefore, this procedure may be utilized for confirmation of the large Barkhausen discontinuity phenomenon.
The large Barkhausen discontinuity phenomenon is observed, for example, in a thin ribbon of iron based amorphous alloy which has been annealed in a twisted state and when used, reformed by applying torsional stress in the direction opposite to that applied in annealing, and in Wiegand wire of an Fe-Co-V alloys and Fe-Ni type alloy having a stress double-layer introduced by working a slender wire. The phenomenon of the discontinuous magnetization of the Wiegand wire is generally called the Wiegand effect, and is substantially the same as the aforementioned large Barkhausen effect (or discontinuity).
Recently, a wire of amorphous alloy having large magnetostriction properties prepared by an in-rotating-water melt spinning method has been found to exhibit extremely sharp large Barkhausen discontinuity as it is formed without further working nor annealing under torsion stress. These alloys have come to be used for various magnetic sensors.
The above examples are based on the magnetization behavior of a magnetic substance with a large magnetostriction in a field of stress including internal stress. On the other hand, a magnetic substance was reported which is almost free from magnetostriction and yet exhibits large Barkhausen discontinuity, in the preprint of the 13-th Meeting of the Magnetics Society of Japan, p. 370. This report discloses that a Co-Fe based amorphous alloy ribbon of 0.8 mm in width, 30 .mu.m in thickness, and 40 mm in length, and having no magnetostriction, came to exhibit large Barkhausen discontinuity after annealing at 300.degree. C. for 30 minutes with a static magnetic field of 100 Oe and subsequent annealing at 300.degree. C. for 30 minutes without application of a magnetic field. This is considered to be due to the fact that magnetic domain walls were formed and fixed in a portion of the thin ribbon by annealing, which controls the movement of other domain walls, resulting in large Barkhausen discontinuity.
Commercial products utilizing the large Barkhausen discontinuity include article surveillance magnetic markers, magnetic sensors, rotation sensors, and the like.
The article surveillance magnetic markers are utilized in surveillance systems in retail stores, libraries, and so forth to prevent unpermitted removal of articles to be sold, or books from libraries. When a marked article is carried out through a monitored area, the surveillance system detects the passing article from a remote place according to the marker preliminarily attached to the article. Among the markers, those employing magnetic substances are widely used in retail stores and other places because of their small size and inexpensiveness, which are suitable for disposable articles. A ferromagnetic substance is incorporated in the marker, while a calling-signal-transmitting coil and a signal-receiving search coil are installed in the surveillance area. When the ferromagnetic substance is magnetically reversed by the specific frequency of cyclic magnetic field of the transmitted calling-signal, a voltage is induced by electromagnetic induction in the search coil. The more rapid the magnetization behavior of the magnetic substance, the steeper the pulse voltage generated, and the higher the order of the harmonic wave generated for the frequency of the calling cyclic magnetic field. The surveillance system utilizes the presence or absence of harmonic waves for detection.
The ferromagnetic substance employed as the marker must be selected so that its emitted signal may be differentiated from the other signals given off by an ordinary magnetic material, such as a shopping basket or a wrist-watch, etc. as distinctly as possible. Therefore, markers are normally made of a material having magnetic characteristics differing greatly from those of ordinary magnetic materials. That is, the material is of high magnetic permeability which magnetically becomes reversed extremely rapidly in response to the applied cyclic magnetic field; or, in other words, as disclosed in U.S. Pat. No. 4,660,025, a material exhibiting large Barkhausen discontinuity.
The examples of markers specifically described in U.S. Pat. No. 4,660,025 are amorphous metal wires and amorphous metal ribbons. The amorphous metal wire is made of an alloy having the composition Fe.sub.81 Si.sub.4 B.sub.14 C.sub.1 (atomic %), being 125 .mu.m in diameter and 76 mm in length. This metal wire is shown to exhibit large Barkhausen discontinuity under a critical magnetic field H* of 1.0 Oe or less and to generate signals discernible to a 99-th harmonic wave in response to an inputted cyclic magnetic field of 60 Hz, which is far sperior to the "METGLAS" (trade name, made by Allied Corporation, Fe-Co based amorphous metal ribbon) and a permalloy ribbon which are described as comparative examples. On the other hand, the amorphous metal ribbon described is subjected to torsion-annealing. For preparation thereof, an amorphous metal ribbon having a composition of Fe.sub.81 Si.sub.4 B.sub.14 C.sub.1 (atomic %), 2 mm wide, 25 .mu.m thick, and 3 to 10 cm long, is annealed under application of torsional stress at 380.degree. C. for 25 minutes, and the resulting ribbon is reformed by holding between films or the like, whereby stress is introduced to induce discontinuous magnetization.
The above-cited U.S. Pat. No. 4,660,025 also describes an apparatus for discerning the marker signals. The discerning apparatus comprises a transmitting section and a receiving section. In the transmitting section, a magnetic field-generating coil is connected to a frequency generator. In the receiving section, a high-pass filter, a frequency-selective detecting circuit, and a warning unit are connected to a magnetic field-detecting coil. In the transmitting section, a cyclic magnetic field of a predetermined frequency is generated as a calling signal. The magnetic field-detecting coil in the receiving section receives from the marker the transmitted signal and the signal containing harmonic waves of the transmitted signal. The signals are filtered by a high-pass filter to eliminate signals of lower frequency, and fed to the frequency-selective detecting circuit. The warning unit is actuated when a predetermined frequency pattern, amplitude, or pulse interval is detected by the frequency-selective detecting circuit.
The magnetic field sensor detects an external magnetic field based on the presence of the critical magnetic field H* which induces the large Barkhausen discontinuity. By application of a magnetic field of H* or stronger, a pulse voltage is generated in a search coil placed near a material exhibiting large Barkhausen discontinuity. From the presence or absence of, or the number of pulses generated, the state of the magnetic field is detected. Thus, magnetic fields stronger than H* and weaker than H* can be detected digitally by two responses: ON or OFF. These types of sensors are characterized by reliable action, even with extremely slow change of external magnetic field that cannot actuate a conventional electromagnetic induction type sensor employing a high magnetic permeability material.
The rotation sensors represent another application field of the magnetic sensor. A measured object is equipped with a magnetic field-applying means such as a permanent magnet. In the vicinity thereof, a sensor is placed which comprises a large Barkhausen discontinuity material and a search coil. The rotation speed is measured by the number of pulse voltages generated in the sensor by the critical magnetic field H*. This rotation sensor is characterized by a broad measuring range of from extremely slow rotation to high speed rotation.
In the prior art, the materials exhibiting large Barkhausen discontinuity are very limited, and the shapes of the materials are classified into thin ribbon types and wire types. Thus thin ribbon type materials, which are made from amorphous metal ribbon to be annealed in a twistes state, needed to be subjected to torsional stress-application in the direction opposite to the twist direction in annealing, when used. The treatment is conducted, for example, according to the description in IEEETRANS. MAG. VOL. MAG-17, No. 6, November, pp. 3370-3372 (1981). In such a procedure, an amorphous alloy ribbon about 1 mm wide and about 0.035 mm thick, having the composition of Fe.sub.80 B.sub.20 (atomic %) in a toroidal shape of 4 to 8 mm in diameter, is annealed at a temperature of not lower than the Curie temperature and lower than the crystallization temperature, and subsequently it is reformed by application of torsional stress in the direction opposite to that in annealing, on use to induce a properties of large Barkhausen discontinuity. By this treatment, a torsional stress of from 45 to 90 kg/mm.sup.2 was applied to the specimen. However, annealing a two-dimensional article like a ribbon under application of torsional stress is not readily practicable industrially. Furthermore, the above-cited reference describes the necessity of reforming the thin ribbon forcibly by some method, which undesirably limits the structure and uses of elements or the like employing the thin ribbon.
In contrast, the materials not requiring stress application are exemplified by Wiegand wires and amorphous alloy wires. The Wiegand wires, however, have been used only in limited applications because the wire requires, for magnetic reversal, a critical magnetic field H* of as strong as several Oe to several ten Oe and the vibration of the critical magnetic field intensity H*, i.e., jitter, is large, resulting in low precision.
In contrast, the amorphous alloy wires have excellent characteristics because the critical magnetic field H* required for magnetic reversal is not stronger than several Oe, and is controllable arbitrarily by the process of preparation. This field is highly precise, with a vibration of critical magnetic field H* of not more than 5%, and the pulse generated in the coil is extremely steep, and the harmonic wave components are remarkably predominant. For this reason the amorphous alloy wires are widely used as a material for article surveillance magnetic markers, rotation sensors, and the like. However, the applications were limited because of the wire-like shape. Further, extreme shortening of the wire makes the demagnetizing field stronger, thereby suppressing the large Barkhausen discontinuity phenomenon, which limits miniaturization. For example, the above-cited U.S. Pat. No. 4,660,025 discloses that the demagnetizing factor must be less than 0.000125, therefore, the amorphous metal wire is required to be in a size range of from 90 to 150 .mu.m in diameter and from 10 to 100 mm in length.
The amorphous metal wires exhibit large Barkhausen discontinuity under no stress application, but application of stress may sometimes suppress the large Barkhausen discontinuity. Accordingly, the wires should be designed to be free from undesired stress insofar as possible. A practical marker is constituted of a sheet of release paper, a double-sided adhesive tape, an amorphous metal wire, and a sheet of protective paper to hold the amorphous metal wire, all being devised to avoid unnecessary stress. It was considerably difficult in many cases to support a special shape of thin thread by avoiding stress.
A thin film configuration is generally understood to be effective in achieving small size and a flat shape. Therefore, a thin film configuration of a material exhibiting large Barkhausen discontinuity, if it could be obtained, could be expected to be used extensively in a wide range of applications.
Use of an organic polymer substrate could result in mass production at low cost with expansion of the field of use. This may be easily understood by following the development of magnetic tape. Further, the thin-film to which tensile or torsional stress must be applied on use were extremely limited in the purpose thereof.
Although there is no prior art clearly disclosing a thin-film material exhibiting large Barkhausen discontinuity, it is possible to produce thus thin-film material using the above described processes, wherein amorphous metal thin ribbon having magnetostriction is heat treated under torsional stress being applied or that having no magnetostriction is heat treated in a magnetic field.
However, for wide use as sensors and article surveillance magnetic markers, the large Barkhausen discontinuity thin film is naturally required to be continuously produced with an inexpensive substrate, without applying stress intentionally on use. A film composed of organic polymers is suitable as the substrate therefor.
On investigation of the aforementioned production methods, it is easily understood that none of the production processes employed a substrate of an organic polymer. The organic polymer substrate is not resistant to the temperature required in heat treatment necessary in any prior art process. In particular, the process comprising heat treatment with torsional stress being applied requires a continuous application of stress on use, with disadvantage.
Accordingly, the development of a magnetic thin film which exhibits large Barkhausen discontinuity, formed on an organic polymer substrate, not requiring intentional stress application on use, and any heat treatment after film formation is desireable, but no such film has ever been produced.
After comprehensive studies to solve the above problems, the inventors of the present invention found that a magnetic thin film which has remarkably large Barkhausen discontinuity characteristics, formed without intentional application of stress, can be produced even by use of an organic polymer material as the substrate. This may be accomplished by forming the thin film by a sputtering method, introducing the thin-film-constituting particles mostly in oblique direction to the substrate, or specifically by placing the substrate in a direction oblique to the target face.